An integrated starter generator system

ABSTRACT

An Integrated Starter Generator System The present invention relates to an Integrated Starter Generator system (100) comprising a battery (110) and a three-phase brushless DC electric machine (130). The electric machine (130) has a stator (132) with 6n stator teeth (132′), ‘n’ being a natural number, and each stator tooth (132′) has a coil corresponding to one of the three phases. The electric machine (130) further has a rotor (134) with 6n±2 rotor poles (134′) facing the stator, and magnets on the rotor poles (134′) are disposed with an alternating sequence of magnet polarity facing the stator (132).

FIELD OF THE INVENTION

The present invention relates to an Integrated Starter Generator system.

BACKGROUND OF THE INVENTION

Conventional ISG systems comprise a three-phase brushless direct-current(BLDC) machine coupled to an engine crankshaft. The BLDC machinesatisfies two purposes—to start the engine by rotating engine crankshaftto sufficiently high speeds to enable self-sustaining combustion, and toact as an alternator once the engine starts.

An electronic control unit (ECU) is typically coupled to the battery andto the BLDC machine. One of the primary purposes of the ECU is to applyoptimal commutations to the BLDC machine to generate positive torqueduring engine starting, and to generate negative torque once the enginehas started. The ECU typically comprises a three-phase H-Bridgecircuitry to achieve the above. The ECU typically uses either 6-stepcommutation, sinusoidal commutation or space vector PWM to actuate thethree-phase H-Bridge to produce required torque. Furthermore, typicalISG systems also comprise position sensors such as hall-effect sensors,to detect the relative position of rotor with respect to stator, andprovide the sensed position to the ECU to perform optimal commutation,the position sensors and ECU typically connected using a wiring harness.Position sensors and the related wiring harness are prone to failurebecause of high temperature, vibration, and proximity to mechanicalobjects rotating at high speeds, therefore presenting a need forsensor-less operation of the BLDC machine. However, BLDC machines usedin conventional ISG systems are not necessarily favourable to be used insensor-less ISG systems.

While acting as a motor to start the engine, the BLDC machine isrequired to generate sufficiently high torque to make speed of enginecrankshaft reach a threshold, wherein self-sustaining combustion ofengine can sustain with high probability. The said threshold istypically higher when ambient temperature is substantially lower thanroom temperature, because of increase in viscosity of oil lubricatingthe engine. Furthermore, the said threshold is typically higher when theengine has not been started for a substantial amount of time. BLDCmachines with high back-emf voltage per unit RPM, pose a challenge togenerate high amount of torque as speed of engine crankshaft increases.As speed of engine crankshaft increases, the difference between batteryvoltage and back-emf voltage reduces, hence limiting the amount ofcurrent that can be provided to the machine. This limit in currentcorrespondingly reduces the torque producing ability of machine, henceadversely affecting engine starting function.

While acting as an alternator, the speed of the BLDC machine reachessubstantially high rotations per minute, resulting in large value ofback-emf voltage being induced in the stator windings of the BLDCmachine, the value of back-emf induced being substantially larger thanthe nominal voltage of battery. Because of the large value of back-emfinduced compared to the nominal voltage of battery, the batteryterminals appear as a short-circuit as perceived by the BLDC machine.Therefore, the amount of current flowing through the machine lines areapproximately equal to the short-circuit current of the BLDC machine.The short circuit current of the BLDC machine is defined as the amountof current flowing through the machine lines if the motor line terminalsare shorted, and the BLDC machine is rotated at high speeds. Theshort-circuit current typically increases with speed and eventuallysaturates. While acting as an alternator, the BLDC machine line currentalso flows through the power switches of the ECU. The line currentsresult in heat generation in stator winding of the BLDC machine becauseof resistance of stator winding, as well as result in heat generationinside the ECU because of the resistance of power switches. Therefore,it is preferable to reduce the amount of short circuit current of themachine to improve system efficiency.

BLDC machines used in conventional ISG systems comprises a stator and arotor. The number of stator teeth and magnets are typically 3n and 2nrespectively where n is a positive natural number. The stator isdisposed with 3 sets of windings resulting in three phases, each phasebeing typically 120 electrical degrees apart from the other.Furthermore, for traditional ISG systems, BLDC machines are typicallychosen such that they have line-to-line back-emf voltage amplitude to begreater than 75% of the nominal battery voltage, when the machine isrotated at 1000 RPM. It is perceived that BLDC machines with highback-emf voltage per unit RPM are well suited for ISG application. BLDCmachine as described above has low value of stator winding inductance,hence resulting in large value of short circuit current. Furthermore,the high back-emf value of the BLDC machine adversely affects the enginestarting function of the BLDC machine as discussed previously.

Thus, there is a need in the art for an Integrated Starter Generatorsystem which addresses at least the aforementioned problems.

SUMMARY OF THE INVENTION

In one aspect of the invention, the present invention is directed at anIntegrated Starter Generator system having a battery and a three-phasebrushless DC electric machine. The electric machine has a stator with 6nstator teeth, ‘n’ being a natural number, and each stator tooth has acoil corresponding to one of the three phases. Further, the electricmachine has a rotor with 6n±2 rotor poles facing the stator, and magnetson the rotor poles are disposed with an alternating sequence of magnetpolarity facing the stator.

In an embodiment of the invention, average width of each stator teeth issmaller than 1.2 times the diameter of the stator divided by number ofstator teeth.

In a further embodiment of the invention, back-emf constant of theelectric machine is substantially between 25% of a nominal batteryvoltage and 75% of the nominal battery voltage.

In a further embodiment of the invention, the battery has the nominalbattery voltage between 10V and 14V.

In another embodiment of the invention, the stator has 18n stator teeth,and the rotor has 18n±2 rotor poles facing the stator.

In an embodiment, n=1, with the stator having 18 stator teeth (132′),teeth numbered 1, 3, 10 and 12 are wound with a coil (A-A′)corresponding to the first phase in the clockwise sense, teeth numbered2 and 11 are wound with the coil (A-A′) corresponding to the first phasein the anti-clockwise sense. Teeth numbered 4, 6, 13 and 15 are woundwith the coil corresponding to the second phase in the clockwise sense,and teeth numbered 5 and 14 are wound with the coil corresponding to thesecond phase in the anti-clockwise sense. Finally, teeth numbered 7, 9,16 and 18 are wound with the coil corresponding to the third phase inthe clockwise sense, and teeth numbered 8 and 17 are wound with the coilcorresponding to the third phase in the anti-clockwise sense.

In another embodiment of the invention, n=1 with the stator having 18stator teeth and the rotor having 16 rotor poles facing the stator. Inan alternative embodiment, n=1 with the stator having 18 stator teethand the rotor having 20 rotor poles facing the stator.

In a further embodiment of the invention, the stator has 12n statorteeth, and the rotor has 12n±2 rotor poles facing the stator.

In an embodiment, n=1, with stator having 12 stator teeth (132′), teethnumbered 1 and 8 are wound with the coil corresponding to a first phasein a clockwise sense, teeth numbered 2 and 7 are wound with the coilcorresponding to the first phase in an anti-clockwise sense, teethnumbered 4 and 9 are wound with the coil corresponding to a second phasein the clockwise sense, teeth numbered 3 and 10 are wound with the coilcorresponding to the second phase in the anti-clockwise sense, teethnumbered 5 and 12 are wound with the coil corresponding to a third phasein the clockwise sense, and teeth numbered 6 and 11 are wound with thecoil corresponding to the third phase in the anti-clockwise sense.

In a further embodiment of the invention, n=1 with the stator having 12stator teeth and the rotor having 10 rotor poles facing the stator. Inan alternative embodiment, n=1 with the stator having 12 stator teethand the rotor having 14 rotor poles facing the stator.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to embodiments of the invention, examples ofwhich may be illustrated in accompanying figures. These figures areintended to be illustrative, not limiting. Although the invention isgenerally described in context of these embodiments, it should beunderstood that it is not intended to limit the scope of the inventionto these particular embodiments.

FIG. 1 illustrates a schematic view of an Integrated Starter Generatorsystem, in accordance with an embodiment of the invention.

FIG. 2 illustrates a sectional view of a three-phase brushless DCelectric machine with 18 stator teeth and 16 rotor poles, in accordancewith an embodiment of the invention.

FIG. 3 illustrates a sectional view of a three-phase brushless DCelectric machine 12 stator teeth and 14 rotor poles, in accordance withan embodiment of the invention.

FIG. 4 illustrates comparative stator teeth magnetic saturation forelectric machines with thick stator teeth, against electric machineswith thin stator teeth, in accordance with an embodiment of theinvention.

FIG. 5 illustrates comparative back-emf voltage waveform of electricmachines with a high back-emf constant and a low back-emf constant, forISG systems comprising battery with 12V nominal voltage, in accordancewith an embodiment of the invention.

FIG. 6 illustrates comparative torque-vs-speed characteristics ofelectric machine with a high back-emf constant as per conventional ISGsystems, against the electric machine with relatively low back-emfconstant, in accordance with an embodiment of the invention.

FIG. 7 illustrates comparative cranking speed during engine startingoperation between electric machine with a high back-emf constant as perconventional ISG systems, against the electric machine with relativelylow-back-emf constant, in accordance with an embodiment of theinvention.

FIG. 8 illustrates comparative short-circuit current as a function ofmachine speed between electric machine with a low stator windinginductance as per conventional ISG systems, against electric machinewith relatively high stator winding inductance, in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an Integrated Starter Generator system.More particularly, the present invention relates to an IntegratedStarter Generator system with an improved starting performance, improvedgenerating performance, and being favourable for sensor-less operation.

FIG. 1 illustrates a schematic representation of an Integrated StaterGenerator system 100. As illustrated in the Figure, the IntegratedStarter Generator system 100 comprises of a battery 110 having a nominalbattery voltage and a three-phase brushless DC electric machine 130powered by the battery 110. The Integrated Starter Generator (ISG)system 100 further comprises of an Electronic Control Unit 120 coupledto the battery 110 and the three-phase brushless DC electric machine130. The Electronic Control Unit 120 is configured to apply commutationsto the three-phase brushless DC electric machine 130 for a starteroperation, and control the current generated by the three-phasebrushless DC electric machine 130 for a generator operation throughpower switches.

FIGS. 2 and 3 illustrate a sectional view of the three-phase brushlessDC electric machine 130 (hereinafter referred to as “electric machine”).The electric machine 130 has a stator 132 and a rotor 134. The stator132 of the electric machine 130 has 6n stator teeth 132′, where n is anatural number and each stator tooth 132′ receives a coil correspondingto one of the phases of the electric machine 130. The rotor 134 of theelectric machine 130 has 6n±2 rotor poles 134′ facing the stator 132,where, as mentioned earlier, n is a natural number. Magnets forming therotor poles 134′ are disposed in a manner that, the rotor poles 134′ arearranged with an alternating sequence of magnet polarity facing thestator 132. For example, a rotor pole corresponding to a first polarityis disposed between two rotor poles corresponding to a second polarityand vice versa.

In the embodiment referenced in FIG. 2 , the stator 132 has 18n statorteeth 132′, and the rotor 134 has 18n±2 rotor poles 134′ facing thestator 132. The coils are connected in suitable series and parallelcombinations, with suitable number of turns in each coil, as per therequirement.

In the embodiment specifically depicted in FIG. 2 , n is equal to 1 withthe stator 132 having 18 (eighteen) stator teeth 132′ and the rotor 134having 16 (sixteen) rotor poles 134′ facing the stator 132. Asillustrated, teeth numbered 1, 3, 10 and 12 are wound with a coil (A-A′)corresponding to a first phase in a clockwise sense, teeth numbered 2and 11 are wound with the coil (A-A′) corresponding to the first phasein an anti-clockwise sense. Teeth numbered 4, 6, 13 and 15 are woundwith the coil corresponding to a second phase in the clockwise sense,and teeth numbered 5 and 14 are wound with the coil corresponding to thesecond phase in the anti-clockwise sense. Finally, teeth numbered 7, 9,16 and 18 are wound with the coil corresponding to a third phase in theclockwise sense, and teeth numbered 8 and 17 are wound with the coilcorresponding to the third phase in the anti-clockwise sense.

In an alternative embodiment, n is equal to 1 with the stator 132 having18 (eighteen) stator teeth 132′ and the rotor 134 having 20 (twenty)rotor poles 134′ facing the stator 132.

In the embodiment referenced in FIG. 3 , the stator 132 has 12n statorteeth 132′, and the rotor 134 has 12n±2 rotor poles 134′ facing thestator 132. The coils are connected in suitable series and parallelcombinations, with suitable number of turns in each coil, as per therequirement.

In the embodiment specifically depicted in FIG. 3 , n is equal to 1 withthe stator 132 having 12 (twelve) stator teeth 132′ and the rotor 134having 14 (fourteen) rotor poles 134′ facing the stator 132. Asillustrated, teeth numbered 1 and 8 are wound with a coil (A-N)corresponding to the first phase in the clockwise sense, and teethnumbered 2 and 7 are wound with the coil (A-N) corresponding to thefirst phase in the anti-clockwise sense. Teeth numbered 4 and 9 arewound with a coil (B-N) corresponding to the second phase in theclockwise sense, and teeth numbered 3 and 10 are wound with the coil(B-N) corresponding to the second phase in the anti-clockwise sense.Finally, teeth numbered 5 and 12 are wound with a coil (C-N)corresponding to the third phase in the clockwise sense, and teethnumbered 6 and 11 are wound with the coil (C-N) corresponding to thethird phase in the anti-clockwise sense.

In an alternative embodiment, n is equal to 1 with the stator 132 having12 (twelve) stator teeth 132′ and the rotor 134 having 10 (ten) rotorpoles 134′ facing the stator 132.

It is understood that typical sensor-less position estimation methodsrely on inductance variation of stator phase windings as a function ofrotor position with respect to the stator. The inductance variation issignificant if magnetic saturation of stator teeth varies significantlywith rotor position. The magnetic saturation of stator teeth as afunction of rotor position can be increased by reducing the thickness ofstator teeth, resulting in increase in the magnetic flux density in thestator teeth. FIG. 4 b illustrates magnetic flux density in the machinesknown in the prior art while, FIG. 4 a illustrates increased magneticflux density in thinner stator teeth 132′ of the present invention ascompared to conventional thick stator teeth, wherein other features ofthe electric machines, such as number of winding turns per stator teeth,are kept the same. The increased magnetic flux density results inincrease in stator winding inductance variation as a function of rotorposition, therefore resulting in the electric machine 130 as per thepresent invention being favorable for sensor-less operation. In anembodiment, to obtain thinner stator teeth 132′ and obtain increasedmagnetic flux density, average width of each stator teeth 132′ is keptsmaller than 1.2 times the diameter of the stator 132 divided by numberof stator teeth 132′.

Reference is made to FIG. 5 , wherein in an embodiment, the battery 110of the ISG system 100 has a nominal voltage substantially between 10V to14V. In an embodiment, back-emf constant of the electric machine 130 issubstantially between 25% of the nominal battery voltage and 75% of thenominal battery voltage, wherein back-emf constant is defined asamplitude of the line-to-line back-emf voltage when the electric machine130 is rotated at a 1000 RPM. Further, it is well established thatamplitude of back-emf voltage of a three-phase brushless DC electricmachine varies linearly with rotating speed of the machine, andtherefore, as the RPM of the electric machine 130 is increased, theback-emf constant will increase accordingly. FIG. 5 illustratesline-to-line back-emf voltage waveforms of two electric machines beingrotated at 1000 RPM in ISG systems with 12V nominal battery voltage. Thewaveform depicted by dotted line represents the electric machine havingrelatively high back-emf constant as used in conventional ISG systems,while the waveform depicted by solid line represents the electricmachine 130 having relatively low back-emf constant in accordance withthe present invention. As shown, the electric machine with high back-emfconstant has amplitude of line-to-line back-emf greater than 9V, that is75% of 12V. The electric machine 130 with low back-emf constant hasamplitude of line-to-line back-emf voltage between 3V and 9V, that isbetween 25% and 75% of 12V.

As illustrated in FIG. 6 , the electric machine 130 with low a back-emfconstant as per present invention can produce higher torque at highermachine speeds, compared to conventional electric machines with highback-emf constant. Torque producing capability of electric machinesreduce as the difference between battery voltage and induced back-emfvoltage reduces. For a given machine speed, the difference, betweenbattery voltage and induced back-emf, is higher in the electric machine130 with low back-emf constant, compared to electric machines with highback-emf constant. This results in higher torque producing capability ofthe electric machine 130 with low back-emf constant as per the presentinvention, especially as machine speed increases.

It is known that starting operation of IC engines requires thecrankshaft of IC engine to be rotated at a reasonable speed. Theprobability of self-sustaining combustion, resulting in engine startingincreases as the cranking speed of IC engine increases. FIG. 7illustrates higher cranking speed achievement when using the electricmachine 130 with low back-emf constant as per the present invention, ascompared to lower cranking speed achievement when using conventionalelectric machines with high back-emf constant. This illustrates that theprobability of engine starting in an ISG system is higher while usingelectric machine 130 with low back-emf constant as per the presentinvention.

It is further known that short-circuit current of the electric machineis a good indicator of efficiency of an ISG system when the electricmachine is acting as a generator. The efficiency of the system is higherif short-circuit current is lower. FIG. 8 illustrates a reduction inshort-circuit current for the electric machine 130 in accordance withthe present invention, in comparison to conventional electric machinesused in ISG applications. Assuming substantially equal back-emf constantof two electric machines, the short-circuit current is lower for theelectric machine 130 with higher stator winding inductance as per thepresent invention.

The flux linkage in stator winding is provided by the maximum value off(a)=∫₀ ^(2π)N(θ)B(θ+α)dθ, where N(θ) is the winding function, and B(θ)is the fundamental component of magnetic field caused by the rotor polesin the air gap. To achieve substantially equal flux linkage in theelectric machine 130 of the present invention, and therefore achieve asubstantially equal back-emf compared to a conventional electricmachine, the number of turns in the electric machine 130 should beincreased compared to the conventional electric machine. Sinceinductance of machine varies quadratically with number of turns, theinductance of the electric machine will be significantly increased andhigher than the conventional electric machine for similar back-emfconstant.

Advantageously, the present invention provides an ISG system with athree-phase brushless DC electric machine with increased magnetic fluxdensity, resulting in increased stator winding inductance variation as afunction of rotor position and rendering the electric machine favorablefor sensor-less operation.

Further, the present invention provides the electric machine with lowback-emf constant where the difference between battery voltage andinduced back-emf is higher, resulting in higher torque producingcapability of the ISG system in a starting operation. The electricmachine of the ISG system of the present invention also achieves highercranking speed.

Furthermore, the electric machine as per the present invention has areduced short-circuit current due to increased stator windinginductance, thereby having an increased efficiency when the ISG systemis acting as a generator. The reduced short circuit current also resultsin reduction of heat generation in the power switches of the ElectronicControl Unit.

While the present invention has been described with respect to certainembodiments, it will be apparent to those skilled in the art thatvarious changes and modification may be made without departing from thescope of the invention as defined in the following claims.

1. An Integrated Starter Generator system (100), comprising: a battery(110); and a three-phase brushless DC electric machine (130) having: astator (132) with 6n stator teeth (132′), ‘n’ being a natural number,each stator tooth (132′) having a coil corresponding to one of the threephases; and a rotor (134) with 6n±2 rotor poles (134′) facing thestator, magnets on the rotor poles (134′) being disposed with analternating sequence of magnet polarity facing the stator (132).
 2. TheIntegrated Starter Generator system (100) as claimed in claim 1, whereinaverage width of each stator teeth (132′) is smaller than 1.2 times thediameter of the stator (132) divided by number of stator teeth (132′).3. The Integrated Starter Generator system (100) as claimed in claim 1,wherein back-emf constant of the electric machine (130), issubstantially between 25% of a nominal battery voltage and 75% of thenominal battery voltage.
 4. The Integrated Starter Generator system(100) as claimed in claim 3, wherein the battery (110) has the nominalbattery voltage between 10V and 14V.
 5. The Integrated Starter Generatorsystem (100) as claimed in claim 1, wherein the stator (132) has 18nstator teeth (132′), and the rotor (134) has 18n±2 rotor poles (134′)facing the stator (132).
 6. The Integrated Starter Generator system(100) as claimed in claim 5, wherein n=1 with the stator (132) having 18stator teeth (132′), wherein teeth numbered 1, 3, 10 and 12 are woundwith a coil corresponding to a first phase in a clockwise sense, teethnumbered 2 and 11 are wound with the coil corresponding to the firstphase in an anti-clockwise sense, teeth numbered 4, 6, 13 and 15 arewound with the coil corresponding to a second phase in the clockwisesense, teeth numbered 5 and 14 are wound with the coil corresponding tothe second phase in the anti-clockwise sense, teeth numbered 7, 9, 16and 18 are wound with the coil corresponding to a third phase in theclockwise sense, and teeth numbered 8 and 17 are wound with the coilcorresponding to the third phase in the anti-clockwise sense.
 7. TheIntegrated Starter Generator system (100) as claimed in claim 5, whereinn=1 with the stator (132) having 18 stator teeth (132′) and the rotor(134) having 16 rotor poles (134′) facing the stator (132).
 8. TheIntegrated Starter Generator system (100) as claimed in claim 5, whereinn=1 with the stator (132) having 18 stator teeth (132′) and the rotor(134) having 20 rotor poles (134′) facing the stator (132).
 9. TheIntegrated Starter Generator system (100) as claimed in claim 1, whereinthe stator (132) has 12n stator teeth (132′), and the rotor (134) has12n±2 rotor poles (134′) facing the stator (132).
 10. The IntegratedStarter Generator system (100) as claimed in claim 9, wherein n=1, withstator having 12 stator teeth (132′), wherein teeth numbered 1 and 8 arewound with the coil corresponding to a first phase in a clockwise sense,teeth numbered 2 and 7 are wound with the coil corresponding to thefirst phase in an anti-clockwise sense, teeth numbered 4 and 9 are woundwith the coil corresponding to a second phase in the clockwise sense,teeth numbered 3 and 10 are wound with the coil corresponding to thesecond phase in the anti-clockwise sense, teeth numbered 5 and 12 arewound with the coil corresponding to a third phase in the clockwisesense, and teeth numbered 6 and 11 are wound with the coil correspondingto the third phase in the anti-clockwise sense.
 11. The IntegratedStarter Generator system (100) as claimed in claim 9, wherein n=1 withthe stator (132) having 12 stator teeth (132′) and the rotor (134)having 10 rotor poles (134′) facing the stator (132).
 12. The IntegratedStarter Generator system (100) as claimed in claim 9, wherein n=1 withthe stator (132) having 12 stator teeth (132′) and the rotor (134)having 14 rotor poles (134′) facing the stator (132).